In many applications, it may be necessary to prevent voltage arcing across electrical contacts, for example, to prevent arcing across electrical contacts of a relay. With respect to inductive loads, such motors, closing of the relay contacts causes a magnetic filed to be generated in the load. At a subsequent time, the relay contacts open, thereby causing the magnetic field to collapse. However, since current flow through an inductor cannot change instantaneously, a back EMF is generated across the inductive load, which causes the voltage across the inductive load to rise rapidly. This rapid rise in voltage (i.e., a voltage spike) may cause an arc to traverse across the relay contacts. Over a period of time, such arcing may cause, for example, deposits on the relay contacts, thereby reducing the effectiveness of the relay contacts.
Referring now to FIG. 5, there is seen a conventional relay circuit 500 having no arc suppression circuitry. Relay circuit 500 includes relay contacts 505 coupled in series with a power supply 510 and an inductive load 515. While relay contacts 505 remain closed, current flows from power supply 510, and through both inductive load 515 and relay contacts 505. When relay contacts 505 are opened, the back EMF generated across inductive load 515 causes the voltage across the inductive load 515 to sharply rise, as shown in the oscillogram of FIG. 6. This increased voltage may cause an arc to traverse across relay contacts 505.
To prevent the occurrence of such arcing, it is known to connect a capacitor (i.e., an arc suppression capacitor) in parallel with the relay contacts. The capacitor provides an alternate path for current flow through the inductive load when the relay contacts open. In this manner, current flowing through the inductive load flows into and charges the capacitor, thereby causing the voltage across the relay contacts to rise more slowly as compared to a circuit having no arc suppression capacitor. Furthermore, to improve the performance of the arc suppression capacitor, it is known to connect a parallel resistor-diode pair in series with the capacitor, as shown in FIG. 8.
It will be appreciated by those skilled in the art that, if the capacitor charges too quickly, a back EMF may still be generated across the inductive load, which may still cause an arc to traverse across the relay contacts. Thus, to ensure proper arc suppression, the capacitor should be chosen to have a large enough capacitance to accommodate the decaying current produced by the inductive load. However, such large capacitors result in increased cost and circuit size.